Internal thread measuring

One of the items we don't seem to stress is measuring internal or female threads. There are thread mikes, wires, and all sorts of tools for male threads. My question is what is the best way to measure female threads particularly in bores of one inch or so. I would use go-no-go gauges but I can't afford the several gauges I would need for most jobs. Let me add that using screws wouldn't work on some of the optical rings I machine measuring 2.1" ID and modulus 0.7 metric or about 43 TPI. Suggestions please.

Yes you CAN afford to get the proper tools for inspection.
Remember: Tooling does NOT cost.....tooling PAYS.
If you cannot absorb the cost of the required tooling for inspection, amortize the cost into the parts run. OR hit the customer with a "NRE" non recurring expense.

Nastydog nailed it.......we have on occasion made our own Go/Nogo plug style gauges for use in shop. Check out your HB for the data needed for gauge making.

Last edited by Spelunker; 09-09-2008 at 04:32 PM.
Reason: felt like it, and wasnt scared to

We make our own all the time, complete with knurled handles and stamped with all of the info just like if you bought one from a tool place. We have quite an interesting collection after many years, probably worthless to anyone but us.

I make my own for plant stuff. Use machinery handbook for min and max pitch diameters. Make one end to the min and one end to the max.
If it was something I'd be selling, I'd spend the money on a calibrated gage. Many good brands out there for a reasonable price. Mercury Gage for one. I think you can even lease gages.
have fun
i_r_machinist

We make all the gages for the stem nuts we thread here. 99% of them are double start left hand Acme in non-standard pitches for the diameter. We just figure the thread info from the handbook, and cut the gage. Works like a charm.

I've never seen an inside thread bore gage. I've seen several tools like the ones made by Starrett and Universal Punch Corporation that can take variable measurements on inside threads, but they're far more expensive than thread plugs.

Somebody really should make one. If you had a three point bore gage and different anvils with gage balls at the end, I think you could take accurate pitch diameter measurements on inside threads.

It's lunchtime (for me) and I just discovered this thread. Cooincidentally I just finished up 4 hours of measuring internal threads with not one, but two of the Starrett pitch measurement gages. I have no idea what the gages themselves cost new but we have had to have some custom pitch anvils made by Starrett this year and a set of anvils cost around $800. -Mike

Internal thread measuring is interesting to me also. I talked to just about every gage maker about it at EASTEC. Most just use GO/NOGO gages. I think some had a method and special probe head for a CMM machine. None of them were inexpensive.

Seems to me that one could rig up some attachable jaws on the ID measuring jaws of a good quality digital caliper. The jaws would have two balls in one jaw and a single ball in the other. With some gentle wiggling adjust the caliper till you get a max reading "under balls" (as it were.) My guess is for a pitch of 43 TPI you would need something like balls of about 0.01312" diameter. Those would be damn small balls. Assuming for a moment that one could rig up something like that to work on a digital caliper... one would still need to do some math to figure out what the correct measurements should be for the pitch diameter of an internal thread.

I think I have seen such a gage but I don't remember where. it uses special "fingers" and one of the "fingers" is connected to the sensing arm of a dial indicator. None of the vendors at EASTEC had such a gage that I remember seeing.

Did a quick check for the name , but could not recall the company. They make a full line of internal thread gages for all types of threads. Works similar to I.D. groove gage using a dial indicator built into the gage. Starrett makes a groove checker that has the look of these gages. If I remember the name I'll post it.

We have a pair of the Starrett 1140's and most of the metric anvils in both full thread and "cone and vee". It is important to know that for measuring pitch, only the cone and vee will work. The full thread anvils are only for checking internal major diameter as far as I can tell. It's a heck of a system! -Mike

We make all the gages for the stem nuts we thread here. 99% of them are double start left hand Acme in non-standard pitches for the diameter. We just figure the thread info from the handbook, and cut the gage. Works like a charm.

We must be in a similar niche. I cut those frequently. In fact, I have to cut a 1 5/8, 2 start internal left hand Acme today in 2 pieces of Manganeze Bronze.

I've made gages many times both almost as good as factory gages but most often the quick and dirty kind I need to get the job out.

One handy tip not covered so far is to make the gage a trifle under the min tolerance, note the size over wires, and determine the actual pitch diameter to be stamped someplace on the plug not crtical to function. Then screw it into a thread under manufacture. Actual fit is determined by measuring side play with a dial indicator. Add the side play to the measured gage pitch diameter and you have a close approximation to the actual pitch diameter. Careful technique in clean conditions can yield repeat zero readings of +/- 0.0002".

Plug gages are "functional fit" devices. They will not deternine thread form, major and minor diameter deviations, lead error, drunkenness, back taper, or other common errors. An oversize pitch diameter coupled with one of these error sources may result in a false acceptance.

There's no replacing knowledge of how to figure thread manufacturing data as an adjunct to the actual machine work. There's always an oddball thread come along and the savvy shop that serves its customenrs efficiently gets the repeat business.

I have an Indi-cal caliper gage. I bought this used and cheap for the B & S indicator that was on it. These are used to measure internal grooves. The tips could be reshaped to measure pitch diameter. You would still have to make a setting ring, but it would allow you to measure deviation from the master and have a real number to use on the compound.
As stated before, you still need the go-nogo gage for QC if shipping parts out.

Dave, that's the stuff. The Starrett thread tools, like Holescreek said, come with "cone & vee" and full thread rolls. You're supposed to use both (need two gages) to measure a single thread.

Each gage is like $1500 or something. Then, a set of thread rolls is like $200 or something. Similar to the cost of a pair of thread rings, is what I remember.

THEN you'd need a thread setting plug to accurately set the gage.

It's really a high high high production setup... cool though, and they DO make one that DOES give you a variables data measurement on ID threads.

I like winger's idea of modifying an Indi-Cal.

I've used those things quite a bit and own 2 of them. With a .0001" indicator, you can actually get repeatable readings of +/- .0002" accuracy in a clean bore. You just have to be super super careful not to bump it.

There really should be an inside thread bore gage built in the style of a Sunnen but with interchangeable gage balls for contacts. Right? Would that be the best setup?

I'm thinking now of Mitutoyo's Digimatic bore gages, the kind with 3 cylinders for contact and tons and tons of measurement pressure :/ You can replace the cylindrical contact anvils on those things, and they have enough range to get in and out of common threads.

internal threads

Back in the stone age we checked internal threads as follows:

1) go / no-go thread gauge, certified for the tolerances for the job at hand

2) to verify thread form, flank angle, etc. etc. we would put into the hole a 'sausage' of replicast or repliplast or whatever. This is an air-curing plastic with minimal shrinkage and had the ability to replicate the finest surface imperfections.

Anyway, this 'sausage' was pressed down into the threads and left to cure. Perhaps 15 minutes or so.

Picked out the replica and bandsawed a thin slice out of it to show the threads like a saw blade. This was examined on a shadowgraph. The surface finish of the thread was also examined by means of this replica.

Did a quick check for the name , but could not recall the company. They make a full line of internal thread gages for all types of threads. Works similar to I.D. groove gage using a dial indicator built into the gage. Starrett makes a groove checker that has the look of these gages. If I remember the name I'll post it.

Brian

Shop I used to work at had sets of these. NOT "MAHR" but something similar. German or Swiss made I believe. The two partners that owned the shop were from Germany, they may have brought the gages with them.
Side note: Only shop I ever worked at where the boss brought cases of beer around after lunch.

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